In pulse-code-modulation (PCM) communication, pulse-amplitude-modulated (PAM) samples are taken at regular intervals from analog voltages or currents and are then converted into digital form by successive comparisons with a predetermined number of voltage or current thresholds. The polarity of these thresholds is chosen to conform to that of the PAM sample, which must therefore be initially determined. With the use of n thresholds, the resulting n amplitude bits of progressively decreasing rank are supplemented by a sign bit preceding the highest-ranking amplitude bit of the group. The number n depends, of course, on the desired degree of accuracy of amplitude coding. PCM telephone systems generally employ eight bits per sample, the seven amplitude bits enabling the coding of 128 different levels.
The initial determination of the sign bit, whose logical value represents the polarity of the sample, is carried out immediately after the leading edge of the sampling pulse, thus at a time when the measured amplitude of the analog signal may be significantly affected by transients. With low amplitude levels, therefore, the polarity of the signal may be misread whereby the thresholds chosen for comparison purposes may be of the wrong polarity. If, for example, the sample is a low negative voltage and is differentially combined with a series of positive threshold voltages, the resulting pulses will all be positive as though the signal were of large positive amplitude.